In plasma spray coating technology, coatings are applied by injecting a powder into a plasma stream where they are heated, accelerated and then impinging the stream upon a surface, the point at which the stream impinges upon the surface or the impingement point, is moved over the surface, by moving the torch body and/or the object. In order to achieve an optimal coating, the angle of the axis of the plasma stream to the substrate surface at the moving impingement point should be maintained perpendicular. In addition, the standoff, the distance between the nozzle of the torch and the impingement point should be maintained substantially constant as the impingement point moves over the surface. An additional requirement for an optimal coating is that the impingement point move across the surface at a substantially constant rate. For simple surfaces of revolution, such as cylinders, conical sections, and annular surfaces, these requirements are met by merely rotating the object and moving the torch at a constant rate along a straight line.
Flame plating by means of detonation using a detonating gun (D-Gun) has been used in industry to produce coatings of various compositions for over a quarter of a century. Basically, the detonation gun consists of a fluid-cooled barrel having a small inner diameter of about one inch. Generally a mixture of oxygen and acetylene is fed into the gun along with a comminuted coating material. The oxygen-acetylene fuel gas mixture is ignited to produce a detonation wave which travels down the barrel of the gun where it heats the coating material and propels the coating material out of the gun onto an article to be coated. U.S. Pat. No. 2,714,563 discloses a method and apparatus which utilizes detonation waves for flame coating. Using a detonation means with suitable coating materials can produce coatings having various characteristics, such as erosion resistant coatings, hard coatings and the like. Similar to plasma coating techniques, the detonation coating techniques require a barrel for the combustible gases and generally for optimal coating applications, the angle of the axis of the existing gases to the substrate surface should be maintained substantially perpendicular. For simple surfaces of revolution, this requirement is met by merely rotating the object and moving the barrel of the gun at a constant rate along a straight line.
For small hollow bodies, the requirements for achieving an optimal coating are particularly difficult to achieve. For example, it is difficult to coat the internal surface of a small diameter cylinder since the plasma torch or barrel of the detonation gun could be too large to be projected into the small diameter cylinder. The difficulty is due in large part to the size of the plasma torch or barrel of the detonation gun with reference to the internal volume of a hollow body. To coat the internal surface of a small diameter cylinder, one has to resort to electrolytic plating or centrifugal cast alloy techniques. However, these techniques may not be suitable for producing hard coatings on the internal surface of small diameter cylinders.
It is an object of the present invention to provide a process for thermal coating the internal surface of a hollow body.
It is another object of the present invention to provide a process for thermal coating the internal surface of a small diameter cylinder using plasma or detonation gun or other thermal spray techniques, electrolytic, electroless, physical vapor deposition (PVD), chemical vapor deposition (CVD), high velocity oxy-fuel (HVOF) or any other conventional technique.
It is another object of the present invention to provide an efficient and cost effective process of coating the internal surface of a hollow body using plasma or detonation gun techniques.
Further objects will become evident from the drawings and in the description of the invention that follows.